Method for producing a continuous web of fibers comprising long natural fibers, and associated apparatus and web

ABSTRACT

The inventive method includes the following steps:
         bringing a plurality of disjointed ribbons ( 32 ) of fibers into parallel, at least one ribbon ( 32 ) containing long natural fibers;   dispersing the adjacent ribbons ( 32 ) through a field of tips ( 60 ) to form a strip ( 62 ) of parallel fibers;   tensioning and stretching the strip ( 62 ) in the field of tips ( 60 ) parallel to an axis of travel (B-B′);   binding the fibers of the stretched strip ( 62 ) to form the web ( 60 ).

The present invention relates to a method for producing a continuous web of fibers comprising long natural fibers.

Such a web is in particular intended to be used with the production of parts impregnated with a polymer matrix, for example for building, automobiles, or any other application.

Natural fibers have many advantages relative to synthetic fibers.

In particular, natural fibers generally have a low density and a relatively low cost and are ecological. Furthermore, natural fibers are renewable and can therefore be produced without exhausting natural resources.

In this context, flax fibers are widely grown and exploited in many fields, in particular to produce composite materials. Flax fibers are extracted from the stem of the flax plant and may be exploited either in the form of bundles of long fibers, or in the form of elementary fibers, or in the form of microfibrils.

The bundles of long fibers are generally made up of an assembly of elementary fibers connected to each other aby a natural cement and have a length that may be comprised between 2 mm and 1,000 mm.

These bundles of fibers have interesting mechanical properties, since their Young's modulus is advantageously comprised between 30 GPa and 50 GPa.

To use the long natural fibers, it is necessary to produce assemblies of fibers for example to form relatively thick plies, fabrics or canvases. These assemblies are the result of a complex and costly development, but have sufficient mechanical cohesion to lend themselves to subsequent shaping operations for parts for example including the superposition and orientation of the assemblies.

However, the methods for producing these assemblies are complex and relatively expensive. That is why in the field of composite materials, it is preferred to use flax tow, i.e., a fiber with a length of between 2 cm and 10 cm that may be used in the context of traditional so-called “dry” methods including the steps of preparation, carding, burling and optionally needlepunching and which will lead directly to mat development without those assembly steps.

However, the nonwoven obtained has insufficient mechanical properties for use to make composite materials. Additionally, the materials obtained are thick, relatively difficult to handle, and have a quite variable thickness.

FR 2,705,369 describes a method for forming nonwoven plies of flax fibers from natural segments of the flax. This method includes a step for establishing a fibrous ply, then connecting the fibers of the ply using natural cements of the flax.

One aim of the invention is to obtain a method for forming a web comprising long natural fibers that is easy to implement, and that makes it possible to obtain a very thin web, with a constant basis weight, at a low cost.

To that end, the invention relates to a method of the aforementioned type, characterized in that the method includes the following steps:

-   -   bringing a plurality of disjointed ribbons of fibers into         parallel, at least one ribbon containing long natural fibers;     -   dispersing the adjacent ribbons through a field of tips to form         a strip of parallel fibers;     -   tensioning the strip in the field of tips parallel to an axis of         travel;     -   binding the fibers of the stretched strip to form the web.

The method according to the invention may comprise one or more of the following features, considered alone or according to any technically possible combination:

-   -   the step for binding the fibers of the strip includes sprinkling         the stretched strip with a solution, impregnating the solution         between the fibers of the strip and drying the strip to form the         web;     -   the sprinkling of the solution is done by spraying drops, or         forming a foam containing a liquid and a foaming agent, the foam         being deposited on the strip;     -   the tensioning step is done between at least one upstream         roller, advantageously having an outer metal surface, and at         least one downstream roller, advantageously made from wood,         rubber or polymer, the speed at which the downstream strip of         the downstream roller is driven being at least two times,         advantageously at least six times, higher than the speed at         which the strip is driven by the upstream roller;     -   the field of tips includes a plurality of bars transverse to the         axis of travel, each transverse bar including a plurality of         tips, the transverse bars advantageously being movable jointly         with the strip;     -   the speed of movement of the transverse bars is lower than the         speed at which the strip is driven by the or each downstream         roller;     -   the surface density of tips in the field of tips is comprised         between 5 tips per cm² and 23 tips per cm²;     -   the method includes, before the bringing in step, a step for         forming the plurality of ribbons by lining with several unitary         ribbons;     -   at least one first unitary ribbon comprises exclusively long         natural fibers, at least one second unitary ribbon         advantageously including additional natural fibers, separate         from the long natural fibers of the first unitary ribbon, and/or         synthetic fibers of natural origin and/or synthetic fibers of         artificial origin and/or mixtures thereof;     -   the standard deviation of the disjointed ribbons is less than         20%;     -   during the bringing in step, the disjointed ribbons are         positioned in adjacent chutes emerging upstream from the field         of tips;     -   in the chutes, the ribbons are compressed laterally by the         converging walls of the chutes;     -   at the outlet of the chutes, the ribbons come into contact         against one another and penetrate one another;     -   the surface density of the web, after the step for binding the         fibers, is less than 500 g/m², advantageously less than 150         g/m².

The invention also relates to an apparatus for manufacturing a continuous web of fibers comprising long natural fibers, characterized in that it includes:

-   -   an assembly for bringing a plurality of disjointed ribbons of         fibers into parallel, intended to receive at least one ribbon         containing long natural fibers;     -   an assembly for shaping a strip of fibers including an assembly         for dispersing ribbons including the field of tips and a system         for tensioning and stretching the strip in the field of tips;     -   an assembly for binding the fibers of the strip to form the web.

The apparatus according to the invention may comprise one or more of the following features, considered alone or according to any technically possible combinations:

-   -   the bringing in assembly includes a guide delimiting a plurality         of chutes positioned side by side, each chute being designed to         receive a ribbon.

The invention further relates to a continuous web of fibers comprising at least long natural fibers, characterized in that it includes a plurality of parallel fibers obtained by dispersion and by tensioning fibers coming from the parallel ribbons, the parallel fibers being connected to each other to form the web, the web having a uniform thickness over its width.

The web according to the invention may comprise one or more of the following features, considered alone or according to any technically possible combinations:

-   -   it has a surface density below 500 g/m2, in particular below 150         g/m2, and advantageously a thickness of less than 1 mm;     -   it has a length 100% greater than its width, the web         advantageously being wound around itself to form a roll;     -   the long original fibers are long flax fibers;     -   the web comprises more than 50 wt % of natural fibers;     -   the web has no synthetic binder binding the fibers to each         other.

The invention also relates to a web that may be obtained using the method described above.

The web according to the invention may comprise one or more of the additional features defined above.

The invention will be better understood upon reading the following description, provided solely as an example, and done in reference to the appended drawings, in which:

FIG. 1 is a view of a flowchart illustrating the main steps of a first method according to the invention;

FIG. 2 is a diagrammatic top view of a first apparatus intended to implement the method of FIG. 1;

FIG. 3 is a front view of a part comprising tips, intended to be used in the shaping assembly of the apparatus of FIG. 2;

FIG. 4 is a view similar to FIG. 2, during the implementation of the method of FIG. 1;

FIG. 5 is a transverse section along plane V-V of FIG. 4;

FIG. 6 is a transverse section, along vertical plane VI-VI of FIG. 4;

FIG. 7 is a view of a web obtained using the inventive method, packaged in the form of a roll;

FIG. 8 is a photograph illustrating the outer appearance of a first web according to the invention;

FIG. 9 is a diagrammatic side view of an assembly for providing parallel ribbons and an assembly for feeding them;

FIG. 10 is a diagrammatic side view of an assembly for shaping a strip obtained from the ribbons; and

FIG. 11 is a diagrammatic side view of a binding assembly for binding the fibers of the strip to form a web.

In the rest of this document, the terms “upstream” and “downstream” are generally understood relative to a direction of travel of an object manufactured continuously.

The first web 10 manufactured using a method according to the invention is illustrated by FIGS. 7 and 8. In the example of FIG. 7, the web 10 is advantageously wound around itself to form a roll 12.

The web 10 according to the invention includes long natural fibers, positioned parallel to each other, and at least one binder maintaining the fibers relative to one another.

In one embodiment, all of the fibers of the web 10 are made up of long natural fibers. Alternatively, part of the fibers forming the web 10 are made up of additional natural fibers, different from the long natural fibers, synthetic fibers of natural origin, synthetic fibers of artificial origin, or a mixture of those fibers.

The long natural fibers are advantageously fibers extracted from plants, in particular flax fibers. Alternatively, the long natural fibers are sisal, jute, hemp, kenaf fibers.

The additional natural fibers are for example chosen from among cotton, wool, silk, sisal, hemp fibers or mixtures thereof.

The synthetic fibers of natural origin are for example chosen from among regenerated cellulose fibers, in particular viscose, cupro and/or modal fibers, alginate viscose fibers, lyocell, PLA (polylactic acid) fibers, and mixtures thereof.

The synthetic fibers are formed from petroleum derivatives or molecules from green chemistry (for example, the ethylene resulting from bioethanol). They are chosen from among polyolefin fibers, such as polyethylene and/or polypropylene, polyester, polyamide, polyimide fibers, and mixtures thereof. They may also be bicomponent fibers formed by a polymer and a copolymer, the polymer and its copolymer having different melting points.

Advantageously, the proportion of long fibers of natural origin in the web 10 is greater than 50% of the total weight of the fibers of the web 10.

Advantageously, the long natural fibers are long natural flax fibers.

The long flax fibers come from the flax plant from the “linaceae” family. These fibers are extracted from the periphery of the stem of the flax by mechanically breaking the stem during a grinding operation, then separating the long fibers, the tows, which are short, coarse fibers, and the other components of the flax.

The long fibers thus obtained advantageously have a length comprised between 2 mm and 1,000 mm. “Long fibers” within the meaning of the present invention means that the fibers are advantageously made up of a longitudinal assembly of elementary fibers bound to each other by a natural cement.

The long fibers generally make up bundles that can reach a length of up to one meter. Part of the long natural fibers of the web 10 has a length greater than 50 cm, and in particular between 50 cm and 80 cm.

The bundles of long fibers generally have a diameter comprised between 10 microns and 100 microns.

The binder binding the different long fibers to each other is advantageously made up of natural cement from the plant, in particular natural flax cements, when the long fibers are long flax fibers.

Alternatively, the binder is a synthetic binder, such as a glue or resin, in particular a latex, and/or a binder of natural origin, other than flax segments, for example starch-based.

According to the invention, the web 10 has a width much greater than its thickness and much smaller than its length.

Thus, the width of the web is for example greater than 30 mm, and in particular comprised between 30 mm and 2,000 mm, advantageously comprised between 100 mm and 2,000 mm.

The thickness is less than 0.1 times its width, and is in particular less than 1 mm, in particular smaller than 50/100^(th) mm, advantageously smaller than 10/100^(th) mm.

The web 10 obtained using the inventive method has a uniform thickness. Thus, the standard deviation of the mean thickness of each longitudinal strip of web 10 formed by a longitudinal strip corresponding to 1/10^(th) of the width of the web 10 is less than 5%, in particular less than 2%, relative to the mean thickness of the web 10.

The mean surface density of the web 10 is low. This surface density is less than 500 g/m2, in particular less than 150 g/m2 or even 100 g/m2, and is advantageously comprised between 30 g/m2 and 100 g/m2.

The surface density is constant moving along the width of the web.

The length of the web 10 is much larger than its width. In particular, the length of the web 10 is greater than 10 m, in particular greater than 100 m. The web 10 can thus be wound in the form of a roll 12 having a diameter larger than 100 mm, advantageously up to diameters of approximately 600 mm.

As illustrated by FIG. 8 and in light of the manufacturing method according to the invention, the fibers of the web 10 are substantially aligned in a longitudinal shaping direction A-A′.

Thus, at least 50% of the fibers of the web 10 are parallel to the longitudinal direction A-A′, which is perpendicular to the winding axis of the web 10, when the web 10 is in the form of a roll 12. Thus, the long fibers of the web 10 are generally positioned parallel to each other and are connected to each other by the binder to ensure the mechanical cohesion of the web 10.

The web 10 has a sufficient mechanical cohesion to allow its handling, in particular during subsequent impregnation and superposition operations to form multidirectional mats. Thus, the web 10 may be grasped by a user's hand and by machine handlers, in particular suction handlers (once impregnated, otherwise it is porous), without undergoing mechanical deteriorations, and while preserving its mechanical strength. The web 10 is self-supporting.

The cohesion of the web 10 is such that the mean break force, taken in the transverse direction perpendicular to the preferred orientation direction A-A′ of the fibers, as measured by Standard ISO 13934-1 for a sample 50 mm wide, is advantageously greater than 0.1N, in particular comprised between 0.2N and 2N, for example around 0.5N. Surprisingly, a web 10 comprising long natural fibers and having both a uniform thickness and a very pronounced orientation of the long fibers, while remaining cohesive enough to be handled, is obtained using the method according to the invention.

To that end, the web 10 is formed from longitudinal ribbons of fibers that are brought into parallel and are dispersed to form a strip of parallel fibers, the fibers of the strip next being bound.

“Ribbon”, within the meaning of the present invention, means a longitudinal element including an assembly of fibers, in particular long fibers. The ribbons are for example obtained by teasing or carding, then individual fibers are tensioned that are then grouped together to form a longitudinal bond.

The thickness of the ribbon is generally approximately the same as its width. It is for example comprised between 0.5 and two times its width.

The ribbon may advantageously be obtained from unitary ribbons that are aligned by superposition on one another. In the illustrated example, the width of the ribbons is for example smaller than 30 mm. The thickness of the ribbons is for example greater than 15 mm and comprised between 10 mm and 40 mm.

The length of the ribbons is greater than 800 m and in particular comprised between 850 m and 1700 m.

The lining operations consist of superimposing at least two unitary ribbons, identical or of different natures, then dispersing them in the field of tips, while keeping them tensioned, before reconstituting a single lined ribbon.

The number of linings is variable. This number is comprised between 1 and 15 linings.

The mass per unit length of the ribbons designed to be used in the method according to the invention is for example comprised between 10 g/m and 40 g/m.

The steps of a first method for manufacturing a web 10 according to the invention are summarized in the flowchart in FIG. 1.

Such a method is for example implemented in an apparatus 14 for manufacturing the web 10 diagrammatically shown in FIG. 2 or in FIG. 4.

As illustrated in FIGS. 2 and 4, the apparatus 14 includes an assembly 20 for providing a plurality of fiber ribbons including at least one long natural fiber ribbon, and an assembly 22 for bringing the ribbons to position them parallel to each other.

In reference to FIGS. 2 and 10, the apparatus 14 further includes an assembly 24 for shaping a continuous fiber strip, from ribbons, and an assembly 26 for binding the fibers of the strip to form the web 10, shown in FIGS. 2 and 11.

The apparatus 14 further includes an assembly 28 for packaging the web 10, in particular to store it in the form of a roll 12.

As illustrated by FIG. 9, the provision assembly 20 advantageously includes an area for storing the tops of ribbons 32.

The ribbons 32 are for example stored in the form of tops, advantageously in creels.

The tops are obtained by pouring the ribbon 32 contained in a can, for example with a diameter comprised between 30 cm and 1 m, in particular between 40 cm and 60 cm. Once the can is full, the ribbon 32 is compressed from the top, the can is removed and the ribbon is bound. The top then has the dimensions of the can with a height comprised between 30 cm and 80 cm, in particular equal to approximately 40 cm. The mass of the top is for example comprised between 15 kg and 40 kg.

For the implementation of the method according to the invention, the ribbons 32 provided in the assembly 20 are advantageously ribbons 32 that have been obtained by lining unitary ribbons, in order to obtain homogenous ribbons 32 in terms of mass per unit length. Thus, the mass per unit length of the ribbons 32 is advantageously comprised between 10 g/m and 40 g/m. The standard deviation between the mass per unit length of the different ribbons 32 intended to form the same web 10 is less than 20%.

In the case where the web 10 is made up of long natural fibers, for example long flax fibers, all of the ribbons 32 are ribbons of long natural fibers advantageously obtained by lining unitary ribbons of long natural fibers.

In the case where the web 10 includes additional natural fibers, synthetic fibers of natural origin or synthetic fibers of artificial origin, at least one ribbon 32 is formed from a unitary ribbon made up of long natural fibers and from at least one unitary ribbon made up of additional natural fibers, synthetic fibers of natural origin or synthetic fibers of artificial origin.

In all cases, the unitary ribbons are each dispersed in a field of tips (not shown), then tensioned and grouped by being superimposed on one another. The number of lining operations is comprised between 1 and 15, advantageously between 2 and 4.

The ribbons 32 are then stored in the form of tops and are positioned on the ground or on a holder. A plurality of ribbons 32 are provided in parallel.

As illustrated by FIGS. 2, 9 and 5, the bringing in assembly 22 includes a guide 40 for parallel distribution of the ribbons 32, intended to feed the shaping assembly 24, and a feed mechanism 42 feeding the guide 40 with the individual ribbons 32.

The feed mechanism 42 includes guide rollers 44 and a pair of feed rollers 80A and 80B each designed to drive each ribbon 32 from the bobbin 30 toward the guide 40.

As illustrated by FIGS. 2 and 5, the guide 40 includes a plurality of chutes 46 positioned in parallel.

The chutes 46 are advantageously positioned adjacent to one another. Each chute 46 is delimited by two side walls 48, and by a bottom wall 49. Each chute 46 is designed to receive an individual ribbon, and distribute it toward the shaping assembly 24 for shaping it in the form of rolls parallel to an axis B-B′ for shaping the web.

Thus, each chute 46 extends longitudinally between an inlet opening 50 and an outlet opening 52 situated across from the shaping assembly 24.

In the example illustrated in FIGS. 4 and 5, the guide 40 includes a plurality of chutes 46 converging from upstream to downstream. The transverse section of the chute 46 across from the inlet opening 50 is larger than the transverse section of the chute 46 at the outlet opening 52.

Thus, each ribbon 32 inserted into the chute 46 is guided so as to converge toward the outlet opening 52 to be in contact with the side walls 48 on either side of the ribbon 32, as illustrated in FIG. 5 at the opening 52.

The incline angle of the axis of each chute 46 relative to the axis B-B′ increases from chute 46 to chute 46 while moving from the axis B-B′ toward the outside of the guide 40. The width of each chute 46 at the inlet opening 50 is at least 10% larger than the width of the chute 46 at the outlet opening 52. This allows natural homogenization of the transverse compacting of the fibrous bundles. At the outlet of the chutes, the ribbons that have been compressed laterally by the converging walls of the chutes will tend toward a slight expansion, which brings them into contact against one another, or causes them to penetrate one another slightly.

The maximum width of each chute 46 at the outlet opening 52 is for example comprised between 10 mm and 40 mm.

The thickness of the side walls 48 is less than 5 mm to limit the separation between the different ribbons 32 during the entry into the shaping assembly 24.

As illustrated in FIGS. 2, 4 and 10, the shaping assembly 24 includes a field of tips 60, intended to disperse the fibers of the ribbons to form a continuous strip 62, shown in FIG. 4, and a system 64 for tensioning and stretching the strip 62 in the field of tips 60.

As illustrated by FIGS. 4, 6 and 10, the field of tips 60 includes a plurality of rows 70 of parallel and transverse tips 72 relative to the axis B-B′ of travel. The rows 70 of tips 72 advantageously comprise between 2 and 16 tips per cm, or between 6 and 40 tips per inch.

The tips 72 have a height greater than the height of the ribbons, and for example comprised between 40 mm and 60 mm.

The rows 70 of tips 72 are supported by individual transverse bars 74, positioned parallel to the axis B-B′. The assembly formed by each bar 74 and its tips 72 is commonly referred to as “gills”.

The successive bars 74 are advantageously placed in contact with one another. Each bar 74 advantageously bears at least one row of tips 70, in particular two rows of tips 70 as shown in FIG. 3.

The tips 72 of a first row 70 are slightly offset relative to the tips 70 of a second row, transversely relative to the axis B-B′.

The rows 70 of the field of tips 60 are advantageously movable along the axis B-B′.

Each bar 74 is moved by helical screws and performs a square movement.

Thus, the bars 74 advance with the strip 62 as far as the end of the field of tips 60. They then return to the beginning of the field of tips 60.

To that end, as illustrated by FIG. 10, the field of tips 60 includes a mechanism 75 for longitudinal movement of the bars 74 along the axis B-B′ between the upstream end position and the downstream end position, over a length L1 along the axis B-B′.

The movement mechanism 75 further includes means for returning each bar 74 from the downstream end position to the upstream end position.

To that end, each bar 74, when situated in the upstream end position, is vertically movable between a refracted return position and an active stitching position of the fibers in the plane of the strip 62.

In the downstream end position, each strip 74 is vertically movable between the active stitching position and a retracted position.

The mechanism 75 moves each strip 74 in the direction of travel of the strip 62 from upstream to downstream while keeping the bar 74 in its active position, then moves each bar 74 from the downstream position to the upstream position in the direction opposite the travel of the strip 62 while keeping the bar 74 in its retracted position.

The length L1 of the field of tips 60 is greater than 80 cm and is in particular comprised between 100 cm and 80 cm to accommodate the long natural fibers and allow their arrangement parallel to the axis B-B′.

As illustrated by FIGS. 2 and 10, the tensioning system 64 includes at least one upstream roller 80A, 80B and at least one downstream roller 82A, 82B that are positioned transversely on either side of the field of tips 60, respectively between the feeder assembly 22 and the field of tips 60, and between the field of tips 60 and the binding assembly 26.

In the example illustrated in FIG. 10, the tensioning system 64 includes a pair of upstream rollers 80A, 80B positioned vertically one on the other. The upstream rollers 80A, 80B are mounted rotating around axes perpendicular to the axis B-B′.

The upstream rollers 80A, 80B advantageously have a metal outer surface, in particular chrome. They have an interstice with a height smaller than the height of the ribbons 32, and in particular smaller than 20 mm, to flatten and pinch the ribbons 32.

The interstice 84 is advantageously placed horizontally across from the tips 70 and horizontally across from the outlet openings 52 of the chutes 46 to allow crushing of the ribbons 32 and the distribution of the fibers from the ribbons 32 in the field of tips 60 through the rows 70 of tips 72.

In the example shown in FIG. 10, the tensioning assembly 64 includes at least two downstream rollers 82A, 82B positioned on one another.

The rollers 82A, 82B are mounted rotating around axes perpendicular to the axis B-B′.

The roller 82A is for example formed by a wood, rubber or polymer cylinder. It advantageously has a diameter larger than that of the roller 82B. The roller 82B has a metal outer surface, for example a chrome outer surface.

The first pair of rollers 80A, 80B and the second pair of rollers 82A, 82B are driven so that the speed of the strip 62 formed in the field of tips 60, taken at the outlet of the downstream rollers 82A, 82B, is at least two times, in particular 6 times, advantageously between 6 times and 20 times, greater than the speed of the strip 62 at the outlet of the upstream rollers 80A, 80B.

This allows tensioning and stretching of the strip 62 to adjust its thickness and basis weight.

As illustrated by FIG. 11, the binding assembly 26 includes a device 90 for sprinkling the strip 62 with a liquid or foaming solution, a space 92 for diffusing the solution on the strip 62 and a device 94 for drying the strip.

The solution is advantageously formed by an aqueous solution. In a first embodiment, the aqueous solution is made up of water. Alternatively, the aqueous solution comprises water and a non-ionic surfactant such as a polyvinyl alcohol, to form a foam.

The proportion of polyvinyl alcohol in the solution is less than 1%.

Alternatively, the liquid solution may include a wetting agent.

In the example of FIG. 11, the sprinkling device 90 includes at least one nozzle 96 designed to eject the solution, in particular in the form of drops, mist or foam, and a transverse body 98 for guiding the solution.

Advantageously, the nozzles 96 emerge in the body 98. The body 98 has a lower opening situated across from the conveyor belt 100 of the strip 62, designed to support and drive the strip 62 between the sprinkling device 92 and the drying device 94.

The drying device 94 includes a heating apparatus 102, for example a convector provided with heating plates, and a suction assembly 104 for the vapors for example including a hood 106.

The intermediate space 92 is situated between the sprinkling device 90 and the drying device 94. Its length is for example comprised between 0.5 m and 2 m.

In this example, the web 10 is packaged in the form of a roll 12. The packaging assembly 28 thus includes an axle or mandrel 110 for winding the web, and means (not shown) for rotating the axle to allow the formation of the roll 12.

In one alternative, the packaging assembly 28 includes a device for distributing a separating sheet designed to be wound jointly with the web 10 to separate two successive layers of web 10 in the roll 12.

The method for manufacturing the web 10 according to the invention will now be described.

As illustrated by FIG. 1, this method initially includes a step 120 for providing disjointed ribbons 32, then a step 122 for bringing the ribbons 32 in parallel in the bringing in assembly 22.

The method next includes the shaping of a continuous strip 62 by dispersing the ribbons 32 in the shaping assembly 24, then a step 126 for fastening the strip 62 to form a web 10 in a binding assembly 26.

The method lastly includes a step 128 for packaging the web 10.

The method is advantageously done continuously, i.e., the steps 122 to 128 are carried out successively, one after the other, continuously, without intermediate stops.

In step 120, a plurality of parallel ribbons 32 are formed, for example by being wound in the form of tops 30 of teased ribbons positioned on a holder, or on the ground, advantageously on creels.

As specified above, the ribbons 32 have advantageously been obtained by lining unitary ribbons, the ribbons 32 for example being made up of unitary ribbons of long natural fibers, or being made up of a mixture of unitary ribbons of long natural fibers with at least one unitary ribbon of additional natural fibers, synthetic fibers of natural origin, or synthetic fibers of artificial origin.

The size of the ribbons 32 is substantially constant, such that the standard deviation between the sizes of the ribbons 32 is less than 20%.

Then, during a bringing in step, a plurality of disjointed ribbons 32 are conveyed in parallel from the provision assembly 20 toward the shaping assembly 24 through the bringing in assembly 22.

The number of parallel ribbons 32 is comprised between 2 and 100, and is in particular comprised between 8 and 15 ribbons, in order to produce webs with a width comprised between 10 mm and 2,000 m.

Then, each ribbon 32 is driven by the feed rollers 44, and is directed toward an inlet opening 50 of the chute 46.

The guide 40 therefore receives a plurality of parallel ribbons 32, each ribbon 32 being received in a respective chute 46. Then, the ribbons 32 are driven through each chute 46 by means of upstream rollers 80A, 80B as far as the outlet opening 52.

During the shaping step 124, different ribbons 32 are first pinched in the interstice 84 between the upstream rollers 80A, 80B to reduce their thickness and disperse them laterally.

Then, the fibers coming from the different adjacent ribbons 32 are inserted into the field of tips 60 while being stretched by the downstream rollers 82A, 82B.

To that end, the bar 74 situated in the upstream end position goes from its retracted position to its vertically deployed position so that its tips 72 pass through the alignments of parallel fibers and form a regular strip with a thickness 62, considered vertically, smaller than the thickness of each ribbon 32.

Then, the bars 74 move longitudinally with the fibers of the strip 62 along the field of tips 60 as far as the downstream tip.

During this transition, the fibers form a strip 62 with a uniform thickness over its width.

Furthermore, in light of the relative speed of rotation of the upstream rollers 80A, 80B and downstream rollers 82A, 82B, the strip 62 is tensioned and is stretched in the field of tips 60 to cause a longitudinal alignment of the fibers in a single direction.

The provision of the fibers in the form of disjointed ribbons 32, the dispersion in the upstream rollers 80A, 80B, the passage in the field of tips 60 and the tensioning of the downstream rollers 82A, 82B make it possible, synergistically, to form a strip with a substantially uniform thickness, with a low basis weight, in particular below 150 g/m2, with a substantially parallel alignment of at least 50% by number of the fibers, or 80% by number of the fibers.

At the outlet of the field of tips 60, the strip 62 is pinched between the downstream rollers 82A, 82B and is brought toward the binding assembly 26.

Before it passes in the binding assembly 26, the stretched strip 62 is not transversely cohesive. Thus, the different fibers making up the stretched strip 62 may be separated from each other by simple manual pressure, when they leave the downstream rollers 82A, 82B upstream from the binding assembly 24.

The stretched strip 62 therefore passes in the binding assembly 24 to strengthen its transverse cohesion.

First, the strip 62 is sprinkled in the dispersion device with a solution designed to activate the binding between the fibers. In a first embodiment, the aqueous solution is sprinkled in the form of droplets, for example in the form of a mist that is guided through the body 98.

In one alternative, the solution is in the form of a foam, and the foam is deposited on the strip 62 through the body 98.

Then, the strip 62 passes in the intermediate space 92, allowing the diffusion of the solution between the fibers by capillarity. In the case where the solution is formed by a foam, the bubbles of the foam break, and the liquid thus formed diffuses through the fibers.

This operation results in partially dissolving the natural cements binding the fibers to each other.

Then, the strip 62 impregnated with liquid enters the heating device 94. The strip 62 is heated at a temperature above 70° C., and in particular comprised between 100° C. and 180° C. to allow the evaporation of the liquid part of the solution.

This evaporation is favored by the suction created by the apparatus 104. The solubilized natural cements resolidify, which reinforces the cohesion between the fibers and in particular ensures transverse cohesion to form the web 10.

In the event the web 10 is made up of less than 50% long flax fibers, the natural flex cements are not necessarily sufficient to provide sufficient transverse cohesion for the web 10. In that case, a small percentage of additional binder, as described above, may be added to the solution.

In the packaging step, the web 10 is wound on itself to form a roll around the mandrel 110. In one alternative, an intermediate sheet is inserted between the different layers of web 10 to avoid deteriorating it.

As specified above, the set of preceding steps is carried out continuously, from the parallel bringing in of the ribbons 32 to the packaging of the web 10 after its formation. The speed of travel of the web during all of the operations is greater than 1 m/minute and is in particular comprised between 1 m/minute and 50 m/minute.

When a sufficient length of web 10 has been wound in the form of a roll 12 around the mandrel 110, the roll 12 can be transported easily and economically to its usage location, for example to produce impregnated panels.

Owing to the invention described above, it is possible to form, very economically, a web 10 including a high quantity of long natural fibers, in particular long flax fibers. This web 10 has a small thickness, and a basis weight below 500 g/m2, in particular below 150 g/m2, or even comprised between 30 g/m2 and 100 g/m2. It further has a uniform thickness over its width and a very pronounced orientation of the fibers, which are substantially aligned and parallel to one another.

In particular, contrary to a flax fiber fabric, the fibers are not twisted, or intertwined. Subsequently, the web 10 does not have any raised portion, which limits its thickness and allows the use of a minimal amount of resin when the panels are manufactured.

Furthermore, the manufacturing method not comprising the spinning step, it is very easy to implement and therefore can be done at a competitive price. The web 10 obtained after the binding step is cohesive and self-supporting, such that it can be transported, without risk of deterioration, which also represents an advantage in terms of cost and handling.

Furthermore, the formed web 10 being made primarily or exclusively with a base of natural fibers, in particular long flax fibers, it is completely biodegradable and may be made without using an artificial binder, using only the natural cements from natural fibers, for example formed by hemicellulose and lignin.

Owing to the invention described above, it is possible to produce a web 10, i.e., a planar assembly of fibers, with a uniform thickness, with excellent regularity, both longitudinal and transverse.

This web 10 is obtained from ribbons 32 or locks of long fibers, in particular flax, having a length greatly exceeding their width, which may be considered an infinite length.

The ribbon 32 results from a complex preparation method that made it possible to extract the long fibers to separate them from the tows, which are short, coarse fibers. These ribbons 32 are for example obtained by teasing or carding.

As specified above, to obtain good transverse regularity, it is necessary to regularize the ribbons 32, which have a thickness generally in the vicinity of their width, and have a circular or pseudo-elliptical section. These ribbons 32 are placed side by side in the guide 40 delimiting a plurality of converging chutes 46, so as to bring them into contact against one another, or to cause them to penetrate each other slightly.

This makes it possible to flatten and interlock the ribbons 32, so as to eliminate the ripples that they form to obtain a transversely regular planar assembly.

The characteristics of the chutes 46, in particular the degree of convergence and the optimal distance between the chutes 46, is optimized based on the size of the ribbons to guarantee that lateral expansion effect.

As specified above, the mass per unit length of the ribbons 32 is high, for example comprised between 10 g/m and 40 g/m. To obtain a good longitudinal regularity, the shaping assembly 24 includes both a system 62 for tensioning and stretching the strip, and field of tips 60 with a length greater than or equal to the maximum length of the long fibers.

Thus, the pair of downstream rollers 82A, 82B having a driving speed greater than that of the upstream rollers 80A, 80B, each head of a fiber pinched between the downstream rollers 82A, 82B is driven at a faster speed than that of the strip 62 formed at the outlet of the intake assembly 22.

To prevent that fiber from itself driving the neighboring fibers with which it is in contact, but the heads of which have not yet reached the downstream rollers 82A, 82B, which would lead to stretching by packets and a very irregular web, the tips 72 of the field of tips 60 block the fibers adjacent to the driven fiber. In this respect, the rows 70 of tips 72 parallel to the field of tips 60, supported by the bars 74, move substantially at the same speed as the driving speed of the strip 62 by the upstream rollers 80A, 80B.

Furthermore, when each downstream roller 82A, 82B has a wood, rubber or polymer outer surface, a fiber pinched between the rollers 82A, 82B that would be retained upstream in the field of tips 60 is capable of sliding between the downstream rollers 82A, 82B without being driven and without being broken.

It is thus possible to obtain a substantially parallel alignment of at least 50% by number of the fibers, or even 80% by number of the fibers, with a low basis weight, in particular less than 150 g/m².

The stretching obtained is regular and avoids breaking fibers.

Advantageously, the surface density of tips 72 in the field of tips is comprised between 36 tips per square inch and 144 tips per square inch, or between 5 tips per cm² and 23 tips per cm².

It should be noted that the method according to the invention advantageously applies for ribbons comprising at least one part with long fibers having a length greater than 20 cm, in particular greater than 50 cm. 

1. A method for producing a continuous web of fibers comprising long natural fibers, at least part of the long fibers of the web having a length greater than 20 cm, the method comprising the following steps: bringing a plurality of disjointed ribbons of fibers into parallel, at least one ribbon containing long natural fibers; dispersing the adjacent ribbons through a field of tips to form a strip of parallel fibers; tensioning the strip in the field of tips parallel to an axis of travel; binding the fibers of the stretched strip to form the web.
 2. The method according to claim 1, wherein the step for binding the fibers of the strip includes sprinkling the stretched strip with a solution, impregnating the solution between the fibers of the strip and drying the strip to form the web.
 3. The method according to claim 2, wherein the sprinkling of the solution is done by spraying drops, or forming a foam containing a liquid and a foaming agent, the foam being deposited on the strip.
 4. The method according to claim 1, wherein the tensioning step is done between at least one upstream roller, and at least one downstream roller, the speed at which the downstream strip of the downstream roller is driven being at least two times, higher than the speed at which the strip is driven by the upstream roller.
 5. The method according to claim 1, wherein the field of tips includes a plurality of bars transverse to the axis of travel, each transverse bar including a plurality of tips.
 6. The method according to claim 1, including, before the bringing in step, a step for forming the plurality of ribbons by lining with several unitary ribbons.
 7. The method according to claim 6, wherein at least one first unitary ribbon comprises exclusively long natural fibers, at least one second unitary ribbon including additional natural fibers, separate from the long natural fibers of the first unitary ribbon, and/or synthetic fibers of natural origin and/or synthetic fibers of artificial origin and/or mixtures thereof.
 8. The method according to claim 1, wherein the standard deviation of the disjointed ribbons is less than 20%.
 9. The method according to claim 1, wherein, during the bringing in step, the disjointed ribbons are positioned in adjacent chutes emerging upstream from the field of tips.
 10. The method according to claim 1, wherein the surface density of the web, after the step for binding the fibers, is less than 500 g/m².
 11. The method according to claim 1, wherein the length of the field of tips is greater than or equal to the maximum length of the long natural fibers.
 12. An apparatus for manufacturing a continuous web of fibers comprising long natural fibers, at least part of the long fibers of the web having a length greater than 20 cm including: an assembly for bringing a plurality of disjointed ribbons of fibers into parallel, intended to receive at least one ribbon containing long natural fibers; an assembly for shaping a strip of fibers including an assembly for dispersing ribbons including a field of tips and a system for tensioning and stretching the strip in the field of tips; an assembly for binding the fibers of the strip to form the web.
 13. The apparatus according to claim 12, wherein the assembly for bringing in the plurality of disjointed ribbons of fibers into parallel includes a guide delimiting a plurality of chutes positioned side by side, each chute being designed to receive a ribbon.
 14. A continuous web of fibers comprising at least long natural fibers, including a plurality of parallel fibers obtained by dispersion and by tensioning fibers coming from the parallel ribbons, the parallel fibers being connected to each other to form the web, the web having a uniform thickness over its width, and in that at least part of the long fibers of the web has a length greater than 20 cm.
 15. The web according to claim 14, having a surface density below 500 g/m2.
 16. The web according to claim 15, having a thickness of less than 1 mm.
 17. The web according to claim 14, having a length 100% greater than its width.
 18. The web according to claim 17, being wound around itself to form a roll.
 19. The web according to claim 14, wherein the long original fibers are long flax fibers.
 20. The web according to claim 14, wherein at least part of the long fibers of the web have a length greater than 50 cm. 